Interpretive Summary: Placental size and efficiency, uterine length, conceptus size and fetal red blood cell development during pregnancy have all be proposed to be associated with uterine capacity. Placental and fetal size have been suggested to be controlled by the early development of the embryo, which has been proposed to be controlled by uterine protein secretion, which in turn, is suggested to be influenced by progesterone secretion by the ovary during very early pregnancy. To test these concepts, interrelationships between fetal size, placental weight, early embryo development, uterine protein secretion, early progesterone secretion, fetal red blood cell development and uterine capacity were measured in two groups of gilts that were altered to remove one ovary and one uterine horn, a surgical method that makes litter size in the gilts a measure of uterine capacity. Plasma progesterone concentrations were correlated with early embryo development and uterine protein secretion. Early embryo development was not correlated with placental weight during subsequent pregnancy. Placental and fetal weights were negatively correlated with uterine capacity, but placental efficiency was not. Fetal weights were correlated with fetal red blood cell development, confirming the negative effect of low birth weight on fetal red blood cell development. Uterine length was not related to uterine capacity. Taken together, these data suggest that reduction in placental and fetal weights may lead to improved uterine capacity. This will be likely to negatively impact fetal red blood cell development and methods are needed to improve this aspect of fetal development.